The invention relates generally to a system and method for delivering a stent. More particularly, the invention relates to a stent delivery system (SDS) and method for delivering a self-expanding stent into a body lumen.
In typical percutaneous transluminal coronary angioplasty (PTCA) procedures, a guiding catheter having a preformed distal tip is percutaneously introduced into the cardiovascular system of a patient through the brachial or femoral arteries and is advanced therein until the distal tip thereof is in the ostium of the desired coronary artery. A guide wire and a dilatation catheter having an inflatable balloon on the distal end thereof are introduced through the guiding catheter with the guide wire slidably disposed within an inner lumen of the dilatation catheter. The guide wire is first advanced out of the distal end of the guiding catheter and is then maneuvered into the patient""s coronary vasculature containing the lesion to be dilated, and is then advanced beyond the lesion. Thereafter, the dilatation catheter is advanced over the guide wire until the dilatation balloon is located across the lesion. Once in position across the lesion, the balloon of the dilatation catheter is filled with radiopaque liquid at relatively high pressures (e.g greater than about 4 atmospheres) and is inflated to a predetermined size (which may be the same as the inner diameter of the artery at that location) to radially compress the atherosclerotic plaque of the lesion against the inside of the artery to thereby dilate the lumen of the artery. The balloon is then deflated so that the dilatation catheter can be removed and blood flow resumed through the dilated artery.
A common problem that sometimes occurs after an angioplasty procedure is the appearance of restenosis at or near the site of the original stenosis in the blood vessel which requires a secondary angioplasty procedure or a bypass surgery. Another occurrence which reduces the success of an angioplasty procedure is that frequently the stenotic plaque or intima of the blood vessel or both are dissected during the angioplasty procedure by the inflation of the balloon. Upon deflation of the balloon, a section of the dissected lining (commonly called a xe2x80x9cflapxe2x80x9d) will collapse into the bloodstream, thereby closing or significantly reducing the blood flow through the vessel. In these instances, emergency bypass surgery is usually required to avoid a myocardial infarct distal to the blockage. Side branches, tortuous vessels, and the more distal arteries have also presented serious difficulties in the PTCA procedure because of the balloon diameter.
Conceivably, the dilatation catheter could be replaced with a perfusion type dilatation catheter such as described in U.S. Pat. No. 4,790,315 in order to hold the blood vessel open for extended periods. However, perfusion type dilatation catheters have relatively large profiles which can make advancement thereof through the blockage difficult, and therefore immediate bypass surgery may be the only means of avoiding an infarct distal to the blockage or possibly even death. Additionally, the inflated balloon of these perfusion catheters can block off a branch artery, thus creating ischemic conditions in the side branch distal to the blockage.
In response, one particular endoprosthetic device, known as a stent, has been developed to prevent restenosis and repair damaged vessel walls. Stents are generally tubular shaped intravascular devices having an expandable or self-expanding structure that is placed within a damaged artery to hold it open. They are particularly suitable for supporting and holding back a dissected arterial lining which could otherwise occlude the fluid passageway there through. The use of stents in non-invasive interventional cardiology has proven to have many advantages, including a net gain in Minimal Lumen Diameter (MLD) of the vessel and reduced restenosis rates.
Stents typically are constructed in one of two general configurations: expandable, and self-expanding. Expandable stents require a mechanical force, such as exerted by a balloon disposed within the stent interior, to increase in diameter. Self-expanding stents are generally constructed of shape memory materials that are biased so that the stent diameter will increase from a reduced diameter maintained by constraining forces to an expanded diameter once the constraining forces are removed, without the action of any external mechanical forces.
Self-expanding stents may be formed in a variety of configurations, and such stents made of coiled wire or springs, braided wire or mesh, and fence-like structures configured in a zig-zag pattern are known in the art. Examples of such of these stents can be found in U.S. Pat. Nos. 4,655,771 (Wallsten); 5,405,380 (Gianotti et al.); 5,709,703 (Lukic et al.); and 5,735,871 (Sgro).
Delivery systems for self-expanding stents are generally comprised of a stent circumferentially surrounding the distal end of a delivery catheter. Due to the narrow passageways within the vascular system and particularly the stenotic regions, stents are generally confined in a reduced radius for delivery to the deployment site. Therefore, it is highly desirable to keep the profile of the catheter as small as possible to minimize the radius of the stent mounted thereon. For delivery purposes, these stents are typically held in a minimal diameter state by some structure such as a sheath. Upon displacement of the sheath, the stent is exposed to self-expand and contact the vessel wall. Once the stent is deployed, the catheter is removed, leaving the stent implanted at the desired location to keep the vessel walls from closing and allowing time to heal. Examples of devices of this type can be found in U.S. Pat. Nos. 5,690,644 (Yurek et al.) and 5,735,859 (Fischell et al.). Another device, as exemplified in U.S. Pat. No. 5,372,600 (Beyar et al.), secures the stent to a catheter without the use of a sheath.
The choice of using a self-expanding stent delivery system instead of a balloon catheter is not without tradeoffs. Stent delivery systems for self-expanding stents using a delivery catheter can have larger profiles, be less flexible, and generally feel more cumbersome than their balloon counterparts. Achieving smooth, even expansion of the self-expanding stent can be difficult, in that some self-expanding stents have a tendency to spring outward to their maximum radius when released. This springing action can cause the stent to jump distally in the artery, which, depending on the amount of distal movement of the stent, can displace the stent from its desired deployment location. Additionally, the unconstrained expansion of a self-expanding


stent can result in the self-expanding stent contacting the arterial wall with some amount of force.
What has been needed and heretofore unavailable is an improved stent delivery system capable of securing and delivering a self-expanding stent on a catheter or other delivery device and smoothly releasing the self-expanding stent so that it smoothly and gradually expands to its full expanded size. The present invention satisfies these needs as well as others.
The present invention is directed to a device and method for delivering a self-expanding stent using a variable-strength sheath to restrain the self-expanding stent, which is particularly suitable for use in coronary arteries to hold vessels open after a balloon angioplasty procedure.
The variable-strength sheath stent delivery system in accordance with the present invention includes a sheath having a first portion and a second portion. The first portion is able to flex in order to traverse tortuous lumens, but has relatively little or no compliance (that would otherwise permit a self-expanding stent within the first portion to expand radially outwardly.) A stent positioned with the first portion can not appreciably expand. The second portion of the sheath has a higher compliance, so that it can permit at least some level of expansion of a self-expanding stent positioned within the second portion. By controlling initial expansion of the stent, the sheath provides a transition between the constrained delivery diameter of the stent and the unconstrained deployed diameter of the stent. Distal jumping of the stent during deployment is prevented. The sheath also tempers the final expansion of the stent as it completely exits the sheath, thereby preventing the stent from expanding with too much force into the vessel walls.
In one embodiment of the invention, the self-expanding stent is forced into a constrained position having a low profile or reduced cross section and positioned within the first (i.e., generally non-compliant) portion of the variable-strength sheath. The sheath and stent are then introduced into a body lumen and advanced to the treatment site. Once the stent is in the desired position, the stent may be slid out of the first non-compliant portion, into the compliant portion, and out of the sheath. The sliding out of the stent may be accompanied by a simultaneous, equal, and/or and opposite withdrawal of the sheath from the treatment site, so that the stent remains distally motionless with respect to the treatment site. As the stent enters the compliant portion, it can begin to expand. When the stent exits the sheath, the stent can fully deploy into contact with the arterial wall and provide structural support thereto.
In an embodiment where the stent is held fixed, the sheath slides proximally relative to the desired treatment location, with the stent remaining still with respect to the treatment/deployment location. As the sheath is retracted, the stent expands progressively, so that the distal portion of the stent expands to the diameter of the more compliant sheath section while the remainder of the stent is constrained at the original diameter by the non-compliant sheath section.
In one embodiment of the invention, the compliance of the second portion varies across the length of the second portion. The compliance or the ability of the material to resist applied pressure without changing dimensions, may be less at a proximal end of the second portion, and greater at a distal end of the second portion. The variation in compliance may be achieved by varying the thickness of the second portion, such as by having reduced thickness at the distal end of the second portion or by changing material or material properties such as orientation or crystalinity. The first and second portions may be formed from the same material, with the second portion having less thickness than the first portion.
In one embodiment, the stent is slid out of the sheath by the use of a deployment device, such as a rod, that pushes the stent out of the sheath. The deployment device may push against the stent""s proximal end to push the stent out of the sheath. The deployment device may also serve to prevent a stent from moving proximally as the sheath is withdrawn in the proximal direction from over the stent.
In another embodiment, the stent is positioned on an elongated device, such as a delivery catheter, and the sheath is placed over the stent and elongated device so that the stent is positioned within the generally non-compliant first portion of the sheath. When the sheath is withdrawn from over the stent, as may be achieve by sliding the sheath proximally along the elongated device, the stent is released and can expand.
The present invention also relates to a method of implanting a self-expanding stent using a sheath according to the invention. A stent delivery system is provided with sheath encircling a self-expanding stent. The stent is secured in a compressed, reduced profile configuration in the first portion of the sheath until deployment. The stent delivery system is inserted into a vessel and advanced to a treatment site. The sheath is axially displaced in the vessel as the stent is slid out of the sheath, so that the stent remains axially stationary with respect to the vessel. As the stent slides into the compliant second portion, the stent begins to gradually expand. As the stent exits the sheath entirely, it deploys against the vessel wall by self-expansion. The sheath is then withdrawn from the vessel leaving the deployed stent in place.